DARIEN -- Three people were hospitalized after an ammonia leak Sunday, July 29 at the Birds Eye plant in Darien -- and this is not the first time the plant has come under scrutiny from federal regulators.

Walworth County sheriff's officials said Sunday night, the Racine Hazardous Materials Team concluded ventilation at the site wasn't complete and would require more time to reduce the ammonia contamination to a safe level.

By 2:30 a.m. Monay, the hazmat technicians notified the Birds Eye administration on scene that the building was at an acceptable level and could be turned over to them. At that point all fire units and hazardous materials teams cleared the scene and the Birds Eye staff took control of the facility.

One worker told FOX6 News the ammonia that seeped into Birds Eye Foods on Sunday morning was so thick you could see it. First responders said it was serious -- approximately 140 employees were evacuated.

"It was a serious leak. It was a major leak. It leaked into an enclosed space, a factory area that it actually creates like, a fog," said Lt. John Ennis of the Walworth County Sheriff's Office.

Officials have not determined the exact cause of the ammonia leak, but it may be linked to an evaporation system on the roof. Fifteen people were sent to the hospital. Three remained hospitalized for treatment as of Monday.

"It's quite toxic to breathe if you get it on your clothes, on your skin. If you breathe it in your throat, it can cause severe burns and even death if there's too much of it. It will take away the oxygen and you won't be able to breathe," said Ennis.

FOX6 News has learned the Birds Eye plant in Darien has been cited for several violations recently. Department of Occupational Safety and Health Administration (OSHA) records show a citation in January and two $10,000 penalties for a failure to control hazardous energy or a proper shut-off for equipment dating back to 2017.

Darien resident Josh Wicyk said beyond the ammonia leak, he is one of a group of residents who want Birds Eye to be a better neighbor.

"I've been watching closely. I go to many meetings and talk to a lot of people," said Wicyk.

Wicyk said many days, there is a strong odor from Birds Eye.

"I just want the smell to stop. I don't wish ill on anybody. I hope the people who are still hospitalized recover. I don't want that on anybody. I don't want anybody to lose their jobs. I just want the company to take responsibility and be a good neighbor," Wicyk said.

OSHA has not yet returned a phone call from FOX6 News. A spokesperson from Birds Eye did not respond to emails for comment.

The incident on Sunday has been reported to OSHA and the Environmental Protection Agency (EPA).